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In chemistry, a dangling bond is an unsatisfied valence on an immobilized atom. An atom with a dangling bond is also referred to as an immobilized free radical or an immobilized radical, a reference to its structural and chemical similarity to a free radical.
When speaking of a dangling bond, one is generally referring to the state described above, containing one electron and thus leading to a neutrally charged atom. There are also dangling bond defects containing two or no electrons. These are negatively and positively charged respectively. Dangling bonds with two electrons have an energy close to the valence band of the material and those with none have an energy that is closer to the conduction band.
In order to gain enough electrons to fill their valence shells (see also octet rule), many atoms will form covalent bonds with other atoms. In the simplest case, that of a single bond, two atoms each contribute one unpaired electron, and the resulting pair of electrons is shared between them. Atoms that possess too few bonding partners to satisfy their valences and that possess unpaired electrons are termed "free radicals"; so, often, are molecules containing such atoms. When a free radical exists in an immobilized environment (for example, a solid), it is referred to as an "immobilized free radical" or a "dangling bond".
A dangling bond in (bulk) crystalline silicon is often pictured as a single unbound hybrid sp3 orbital on the silicon atom, with the other three sp3 orbitals facing away from the unbound orbital. In reality, the dangling bond unbound orbital is better described by having more than half of the dangling bond wave function localized on the silicon nucleus, with delocalized electron density around the three bonding orbitals, comparable to a p-orbital with more electron density localized on the silicon nucleus. The three remaining bonds tend to shift to a more planar configuration.
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